1. Field of the Invention
The present invention relates generally to software defined radio systems, and particularly to RF interfaces.
2. Technical Background
A software-defined radio (SDR) ideally allows all of the radio parameters to be programmed dynamically by its computer software. A radio system typically includes components such as mixers, filters, amplifiers, modulators/demodulators, detectors, etc. In the standard radio, these components are implemented exclusively in hardware and their operational settings are predetermined and permanent. In an SDR, the operational settings may be adjusted by the software such that the SDR may be used for a variety of different purposes and over more than one type of wireless network. Accordingly, SDRs may be used in a variety of applications including military, emergency services (e.g., police, fire, ambulance, etc.), commercial and civilian. For example, the various branches of the armed services (such as the Army and Navy) may operate at different frequencies using different types of communication formats. The SDR provides a given user with the ability to dynamically change the SDR such that inter-service communications are effected. In another example, the networks operated by various cell phone providers may use different frequencies or formats; an SDR will dynamically reconfigure itself as the user moves from one network into another.
As shown in FIGS. 1(a)-1(d), SDRs may be implemented using a variety of different form factors. FIG. 1(a) shows an SDR 1000 implemented as a typical cell phone having a speakers 400, microphone 401, display 700, and a keypad input 800. FIG. 1(b) shows another SDR example that may include, e.g., a camera 850, in addition to microphone/speaker 400, keypad 800 and display 700. FIG. 1(c) depicts another SDR form factor. This embodiment is generically referred to as a personal digital assistant and may provide the user with cell phone service, wireless internet access, a camera, digital video, or other such services. FIG. 1(d) is an example of a commercial device that includes a camera 850, a bar code reader (not shown), keypad and data entry components (800, 802), as well as display 700. Those skilled in the art will understand that the present invention should not be construed as being limited to the examples provided herein.
In recent years, innovations in both circuit architectures and process technologies have enabled great programmability with respect to bandwidth, oscillation frequency, gain, and modulation type. However, the interface between the antenna and the receiver which could include an RF LNA, matching network and/or an RF-band filter (often a SAW filter), remains very difficult to tune.
Ideally, the antenna interface of an RF receiver should perform three functions: (1) match the impedance of the antenna so as to extract the maximum possible wanted (in-band) signal power from the antenna and prevent reflections, (2) amplify the wanted signal with low noise, and (3) reject unwanted (out-of-band) interferers. However, the structures currently used to achieve both good impedance matching and low susceptibility to blockers requires resonant structures that are inherently highly frequency dependent. Thus, one drawback to the current state of the art relates to the difficulty in achieving the aforementioned goals over wide RF tuning range.
One of the approaches considered for realizing a wide band receiver (i.e., capable of capturing several widely spaced bands) employs multiple narrowband front-ends that are disposed in parallel. Only one of the parallel receivers may be used at a time. Another approach that has been considered is the use of a wideband receiver that has only moderate rejection of interference (out-of-band IIP3 of <0 dBm) in the various frequency bands. The former approach comes at significant cost in area both on chip and off, and the latter simply cannot achieve the necessary performance for many applications (cellular, etc.). Accordingly, the current state of the art does not provide a high performance, high tuning range SDR.
The architecture of high performance (and therefore) narrowband direct conversion receivers typically includes (in order of the input signal path) an off-chip RF-band filter, a matching network, low noise amplifier (LNA), mixer, and baseband circuitry. The components which are difficult to tune across frequency are the ones which “see” the RF signal; i.e., the components in the signal path that are disposed upstream of the mixer. As those of ordinary skill in the art will appreciate, an RF-band filter rejects out-of-band blockers, and is typically implemented with high-Q off-chip components such as SAW filters. The matching network, typically implemented with a resonant LC network, transfers as much power as possible to the LNA. The LNA absorbs the RF power and provides amplification of the signal with as little noise as possible. Indeed, a good definition of an LNA is an amplifier that provides an impedance match with less than a 3 dB noise figure (something a simple resistive matching network cannot achieve). When the second approach is employed (i.e., a widely tunable receiver), the RF-band filter is removed entirely; a lower performance wideband LNA is substituted in its place. The LNA is matched such that higher power is transferred vis à vis the first approach considered.
In principle, a direct conversion receiver does not require any RF components other than a mixer and local oscillator in order to function. Indeed, early homodyne receivers included only these components; the antenna was directly connected to the mixer without employing an RF LNA. However in order to provide antenna impedance matching, this approach requires additional components between the antenna and the mixer. As noted above, LC impedance matching circuits are frequency dependent and, therefore, narrow band in nature. Additionally, this approach does not provide any rejection of out-of-band interferers.
What is needed therefore is a wide-hand programmable software-defined radio (SDR) receiver that addresses the drawbacks associated with the approaches described above. In particular, what is needed is an SDR that addresses the issues related to impedance matching, noise figure and rejection of out of band interferers.